Phase shift masks are gaining attention as the next generation lithographic technique for microelectronic fabrication due to their capability to produce higher resolution images compared to the conventional binary photomasks. Among the several phase shifting schemes, the attenuating embedded phase shifter proposed by Burn J. Lin, Solid State Technology, January issue, page 43 (1992), the teaching of which is incorporated herein by reference, is gaining wider acceptance because of its ease of fabrication and the associated cost savings. There have been a number of variations associated with this scheme to improve the optical properties of the photomask, i.e. tunability of the optical transmission and resistance against photon irradiation and chemical treatments.
The attenuated phase shift mask (APSM) described in U.S. Pat. No. 5,897,977 to Carcia et al. consists of alternating layers of optically transmissive materials and optically absorbing materials. The advantage claimed by this process is that the phase shift and transmission can be controlled easily by adjusting the thickness of either or both of the layers. However, the deposition process is complicated since two different materials need to be deposited in an alternating sequence, which will increase the cost of the process and increase potential defects in the mask. Also, due to the different etching properties of the two materials, obtaining a smooth line edge by etching is difficult.
The APSM described in U.S. Pat. No. 5,939,227 to Smith consists of a multilayer of SixNy and metal nitride. The advantage claimed by this process is both materials are chemically stable and etch selectivity is well defined. However, the deposition of this process is complicated since it requires two separate targets and a planetary sample stage, which increases the cost for manufacturing and again significantly increases potential defect levels.
A Zr-based APSM described in U.S. Pat. No. 5,935,735 to Okubo et al., wherein the film had 2 to 15% transmittance and less than 30% reflectivity. The Zr-based film consisted of two or more multilayers with different optical properties to achieve the above transmittance and reflectivity. While this scheme provides good tunability, a multilayer structure may not provide good manufacturability. Also, due to the highly stable nature of Zr compounds, the RIE etch selectivity is inferior.
The APSM described in U.S. Pat. Nos. 5,942,356 and 6,153,341 to Mitsui et al., consists of molybdenum, silicon and nitride. The advantage claimed of this process is that it consists of a single material which gives good etch properties. Also, the material is stable during laser irradiation and acid treatment. However, the tunability of % T is not as flexible as the multilayer materials. Mitsui et al., does not incorporate a post-deposition treatment in their patent.
Herein we describe a method to fabricate, through deposition and post deposition treatment, a phase shift photomask that has tunable optical transmission, coupled with stable optical properties during usage (photon exposure and chemical treatments) of the photomask.
The present invention provides a phase shift photomask with a small surface roughness and low defect density by reducing the particulates produced during the deposition process.
We have also discovered that when the phase shift mask or phase shift mask blank is annealed to a temperature higher than room temperature under an atmosphere which contains oxygen at more than 10−3 torr partial pressure, a film structure can be produced that is stable against photon irradiation and chemical treatments for photomask fabrication.